This invention relates to fuse assemblies in electrical protective equipment such as fuse cutouts and, particularly, to such devices for relatively high voltages and currents.
A cutout is a device to isolate distribution circuit branches. It includes a conductor with a fuse in series with an individual circuit branch so that a fault on that branch will cause operation of the fuse without affecting other parallel branches. Representative examples of available cutouts are contained in Westinghouse Descriptive Bulletins 38-631, 38-651, and 38-671 and U.S. Pat. No. 4,480,245, Oct. 30, 1984, by Carothers et al., which are herein incorporated by reference for general structure of cutouts that may be improved in accordance with this invention.
The fuse link itself is located in a fuse assembly, that is a replaceable element of a cutout, that includes top and bottom contact assemblies and a porcelain insulator holding but electrically isolating the top and bottom contact assemblies. Many such cutouts are presently in use and it would be highly desirable if existing cutouts could accept a fuse assembly of the same size and shape but with, where desired, a higher voltage or current rating.
The problem addressed arises because existing fuse assemblies are generally restricted to being capable of interrupting currents only within a specified range. For example on 200 ampere, high voltage (27 kv and higher), distribution cutouts, problems are sometimes encountered in interrupting low currents per Test Series 5 of ANSI C37.41-1981, Table 3. This standard requires two tests using the minimum link rating (e.g., 140 k for a 200 ampere cutout) at a current (378 to 462 amperes for a 140 k link) which will melt the fuse link in approximately two seconds. On low current interruption, the interruption takes place inside a fiber tube of the fuse assembly. On high voltage (e.g. 27 kv) cutouts, the fuse link is located deep within a fuse tube. As the fuse link melts, gas is generated inside the fuse link to blow the pigtail out of the fuse tube and to interrupt the arc. However, with a long (such as 14 inches long) fuse tube for the higher voltage systems and a large pigtail for the higher current rating fuse link, the gases become stagnated inside the tube and interruption becomes extremely difficult. The physical size of the pigtail blocks the gases from being expelled easily and also makes removal of the pigtail more difficult, even though the pigtail is usually connected to a spring loaded flipper as described in the above-mentioned Descriptive Bulletins. It is also, of course, not desirable to have to modify the overall cutout design in order to enable the cutout to handle higher currents or voltages.
Fuse devices are known that include use of a compressed dielectric fluid for functions such as to help interrupt the current after fuse operation. These include, for example, Ackermann 3,265,838; Aug. 9, 1966; and Link 3,771,089; Nov. 6, 1973.
Ackermann presents the general object of providing a fuse construction suitable for interrupting currents over a wide voltage and current range. It utilizes the escape of a dielectric fluid from a capsule not only to effect arc extinction, but also by jet action to effect actual movement of the capsule to a remote position. This produces rapid separation of the fuse terminals and also, where desired, unlatches an associated dropout mechanism. The embodiment of Ackermann, FIGS. 3-5, includes a gas cylinder 8 having a tubular blast tube 7 extending to a point at the lower end of the fuse assembly in which the fuse link 10 is located, as depicted in FIG. 3 of the patent to Ackermann. In FIG. 4 is shown the structure after the fuse has started to burn through and the gas cylinder and blast tube have started to elevate. In FIG. 5 is shown the result after the dropout actuating pin protruding from the gas cylinder has extended through the top of the fuse assembly to enable the dropout mechanism.
In the Link patent, FIG. 1, a fluid 24 in a gas cylinder 32 is released upon burn through of the fuse link 22. The fluid is released into a space defined by arc chutes 38 and is there confined.
The present invention takes advantage of the arc extinction and contact separation capabilities of a pressurized gas cylinder in a manner that allows direct replacement of the instant invention fuse link for those formerly used without gas cylinders and with increased voltage and current rating capability.
In particular and by way of example, the fuse assembly includes within an exterior enclosure, such as of impregnated fiber glass, an interior fiber tube or liner, preferably containing arc extinguishing materials released upon fuse operation. A fuse link has on one side of it a conductor, such as one including a rigid conductive connector and a stranded copper pigtail in series, joined at one end (such as the lower end) to a spring loaded flipper element. The fusible element is connected from the connector to a conductive insert that sits on top of the interior fiber tube. There is also a strain element connecting the connector and the insert so that the assembly is held in place absent operation of the fuse.
A gas capsule, cylinder, or cartridge containing a pressurized insulating gas such as CO.sub.2 or SF.sub.6 is located in the upper portion of the fuse assembly. It rests against the conductive insert and at the top is secured by a spring between the top of the cartridge and a cap that closes the tube. When the fuse link melts, the lower tip of the cartridge burns through. This releases the gas into the arc area, extinguishes the arc and expels hot conductive gases out of the lower end of the tube. The melting of the fuse link also releases the strain element by melting it and causes the spring loaded flipper element to pull out the stranded pigtail and connector, thus further accelerating arc extinction.